More and more, sensors are used in vehicles for detecting environmental amounts, environmental influences, etc., wherein the number of sensors in a vehicle increases continuously, particularly for security-relevant systems. The used sensors serve, for example, for determining ambient pressure, acceleration, revolutions per minute or relative distance or relative movements, respectively, such as the distance to an object close to the vehicle.
Here, the sensors communicate with central control units in a vehicle, such as an on-board computer or an associated electronic control unit ECU, via the usual sensor supply networks in vehicles. The control units evaluate the sensor data received via the sensor supply network and then cause actuation, switching-on or powering-on of a security system in a vehicle. These security systems can comprise, for example, ABS (ABS=anti-blocking system), traction control, airbag system, distance control or other sensor systems. In vehicles, sensor supply networks are normally realized as two-wire connections.
One of the most important and also most widespread security systems are airbag systems. An airbag system, for example with steering wheel airbag, passenger airbag, side airbag, etc. consists thereby mainly of one or several airbag sensors, associated electronic control unit, a trigger arrangement with trigger circuit and the airbag itself.
A sensor that can detect pressure or acceleration, such as a side airbag sensor, is, for example, in a door of the vehicle or at a supporting column of the vehicle and is connected to the electronic control unit via cable and plug connectors. The electronic control unit receives a signal from the side airbag sensor via the sensor supply network, evaluates the same and decides about triggering the airbag. The voltage supply of airbag sensors is also performed via the sensor supply network of the vehicle. Thereby, voltage variations, such as short-term setbacks of the supply voltage, can occur in the sensor supply networks of vehicles and particularly in certain areas of the sensor supply network. For example, heavy jerky movements, such as shocks or vibrations of the vehicle can cause short-term interruptions at one of the plug connections, so that short-term setbacks of the voltage supply of security-relevant systems, such as the side airbag sensor, can also occur. These setbacks of the voltage supply are generally referred to as microbreaks.
For that reason, an effort is made to implement the security-relevant systems in vehicles such that they are as insensitive as possible against such setbacks of the supply voltage, so that an operation of the security-relevant systems, which is as stable as possible, can be maintained, even when such setbacks or breaks of the voltage supply occur.
When microbreaks occur, it can be observed that unfavorably side airbag sensors can be reset unintentionally by such interruptions of the supply voltage, wherein such an unintentional reset (power on reset) would have the effect that the side airbag sensor would run through a complete initialization for a longer time period. Such an initialization includes, among others, a long or extensive start message transmission defined in a protocol. During the transmission of the start message, triggering of the airbag is not possible, since the airbag sensor runs through its initialization in this phase, and can thus provide no measurement data, which would allow the electronic control unit to detect a release event, such as an accident.
Thus, in order to increase the operational safety of a vehicle, it is necessary that the airbag sensor remains operable during such microbreaks, i.e. during short-term setbacks of the supply voltage, for a time period that is as long as possible, or remains uninfluenced by such microbreaks, respectively, so that the whole airbag system is not put out of operation or reset by these microbreaks, respectively.
So far, this increase of operational safety is realized by carrying out the voltage supply from an additional spare voltage source with a battery or a buffer capacitor connected in parallel on the input side.
Frequently, so-called buffer capacitors are used as spare voltage sources, which are to stabilize the operating voltage of the sensor and to maintain the voltage supply of the sensor, while the connection between the electronic control unit and the side airbag sensor is interrupted. Such buffer capacitors are connected on the input side to the supply voltage terminal of the side airbag sensor and are thereby charged to the current operating voltage, i.e. the operating voltage currently applied to the sensor. When the difference between the specified bottom limit of the operating voltage and the reset threshold (reset threshold) is low, the buffer capacitor is only charged to a voltage level, which is slightly above the reset threshold, when the operating voltage is already close to the bottom limit. When the supply voltage is set back to a value lying below the reset threshold, the voltage supply can be maintained by energy stored in the buffer capacitor, whereby a buffer capacitor can provide only relatively little energy for shunting the microbreak in the above illustrated case.
In that context, it should be considered that only relatively little of the stored energy or charge, respectively, can be drawn from a buffer capacitor, or the buffer capacitor can only be discharged across a relatively low time period, until a reset is triggered at the sensor, since the difference between the two voltage states, namely the lower operating voltage limit and the reset threshold is relatively low. Thus, the difference between the two charge states of the buffer capacitor, namely the charge state and the lower operating voltage limit and the charge state at the resent threshold is relatively low. Thus, reliable voltage supply of side airbag sensors is often not possible when variations of the operating voltage (microbreaks) occur.
A further conventional procedure for supplying electronic circuits during a microbreak of the supply voltage as stable as possible with energy, is to use a so-called battery backup switch. Here, a supply voltage terminal of the side airbag sensor is connected to the electronic control unit via the battery backup switch, wherein the battery backup switch switches to a backup battery connected to the battery backup switch, when the input voltage provided by the electronic control unit or the sensor supply network breaks down. The battery backup switch switches between the supply voltage provided by the electronic control unit and the battery voltage, such that the voltage source, i.e. the sensor supply network or the backup battery with the higher voltage provides the current supply voltage to the side airbag sensor. Such battery backup switches are sold for example, by the company “Analog Devices” with the type designation “ADM 690”.
It is a disadvantage of the usage of a battery backup switch that the used batteries only have a relatively limited life span, and thus have to be replaced after a certain time period, such that a power supply system with backup battery is expensive to implement and thus relatively expensive. Above that, it should be considered that batteries themselves are relatively sensitive to external environmental influences and particularly with regard to the ambient temperature, wherein batteries frequently loose the utilizability very fast with very low temperatures. Thus, the reliability of security-relevant sensor systems using battery backup switches is also limited. For that reason, generally, the usage of battery backup switches for security-relevant sensor systems in vehicles is avoided.
Thus, the conventional power supply arrangements have shown to be problematic in allowing a reliable provision of a stable, predetermined output signal with a predetermined output signal level with undesired vibrations of the input signal level in an efficient way, particularly when the input signal level falls below a critical threshold, based on energy stored in a buffer capacitor or another additional backup battery.